1. Field of the Invention
This invention relates generally to power supplies utilizing dc/dc converters with over-voltage protection and more particularly to over-voltage protection for power supplies for supplying power to plasma with a shunt switch across the output of the power supply.
2. Brief Description of the Prior Art
Most DC/DC power supplies use an output filter stage to reduce the ripple on the output voltage. This filter usually consists of inductors and capacitors both of which store energy. When this power supply is used to deliver power to a dc plasma process, in the event that an arc occurs, as described in the copending patent application Ser. No. 10/884,119 filed Jul. 2, 2004 entitled “Apparatus and Method for Fast Arc Extinction with Early Shunting of Arc Current in Plasma” assigned to the assignee of the instant application and incorporated herein by reference, this energy can be delivered to the plasma arc which is detrimental to the process. To reduce energy delivered into an arc, a shunt switch is used to circulate the inductor current inside of the power supply. When the arc is extinguished, the shunt switch opens. The energy stored in the inductor when it is released to the plasma, can generate a very high output voltage with very high dv/dt that can damage the power supply or the chamber. In order to protect the power supply different over-voltage protection circuits have been used. Simpler over-voltage protection scheme designs are based on energy being dissipated on passive components such as RC snubber circuits, transient voltage suppressor circuits, or metal oxide varistors. More complicated designs transfer the energy back to the input bus.
In typical switch mode power supplies that utilize a dc—dc converter with a shunt switch SW2 such as that shown in FIG. 1, the output stage can be approximated with a two pole output filter that comprises an inductor L and capacitor C as shown. In this configuration the energy from the output capacitor C will be dumped into an arc. The goal of power supply designers is to minimize the size of the output capacitor. As a result the inductor has to be increased in order to keep the output voltage ripple low. Energy stored in the inductor can be easily controlled by shunt switch SW2. Closing the switch during an arc allows inductor current to circulate inside the power supply through SW1 in the p2 position and shunt switch SW2. If there are small losses on the switches then the inductor current will not decay appreciably during this interval. At the moment the power supply is restarted by opening the shunt switch, the inductor energy will start charging the capacitor and can make the output voltage very high. To protect the switch SW2 the over-voltage circuit 10 is required.
There have been many patents that teach over-voltage protection for dc power supplies. For applications in dc sputtering processes, see for example U.S. Pat. No. 5,584,974 issued to Jeff Sellers on Dec. 17, 1996 that discloses over-voltage detection and clamping circuit that comprises a string of zener diodes or equivalent voltage limiting devices connected to the applied voltage. This circuit absorbs the voltage excursions beyond the threshold and protects the power supply and the substrate in the plasma chamber. As a result, most of the inductor energy is dissipated across the zener diodes, and in turn limits the number of arcs per second that this power supply can operate with.
It would be desirable if there were provided a power supply utilizing a dc—dc converter with an over voltage protection for the shunt switch that is used to minimize energy delivered to an arc. and limits the voltage to the ignition voltage level when the power supply extinguishes an arc.